Highlights from 6th Cluster Workshop

Magnetic reconnection is a fundamental process in space and astrophysical plasmas through which plasmas of different origin are able to mix and become accelerated into energetic jets, and which allows the transfer of energy between different regions of space. Key examples of magnetic reconnection in action can be found in galactic accretion discs, flares on the Sun, and within the terrestrial magnetosphere, the magnetic "bubble" that shields the Earth from the direct effects of the solar wind blowing from the Sun. Energy tapped from the solar wind as a result of the magnetic reconnection process, is stored within the magnetosphere, and, when released, causes the fantastic auroral displays familiar to those who live in northern climes.

This and many other burning scientific issues were discussed by about hundred scientists who gathered together for the 6th Cluster Workshop that was held from 29 September to 3 October 2003, at the European Space Research and Technology Centre (ESTEC) in Noordwijk, The Netherlands. The workshop primarily dealt with recent observations of the ESA's Cluster mission in the dayside magnetosphere, with the aim of providing a coherent view of some of the key questions about the structure and dynamics of the dayside magnetosphere.

Orbit of Cluster (red ellipse) drawn over a sketch of the Earth's magnetopshere. Key regions of the dayside magnetosphere are the bow shock (1) and the magnetopause (2) that are frequently encountered by the four Cluster satellites.

The magnetosphere has been investigated with space probes for some 30 years so that a general picture of the magnetosphere can be drawn. However, many details of how the terrestrial environment responds to variations in solar radiation and solar wind is still open (this is also called the "space weather"), and what the implications of changing and extreme space weather conditions are to humans and their technologies. Understanding and predicting space weather requires multi-point measurements in the magnetosphere at various critical locations. The Cluster mission, consisting of four satellites, cannot solve all the problems but it provides us with the first real 3-D measurements in space so that spatial and temporal features can be distinguished.

The solar wind is a supersonic stream of electrons and ions emerging from the Sun. The Earth posseses a strong internal magnetic field that prevents the solar wind from hitting the upper atmosphere (as it is the case at Venus and Mars). Instead, the solar wind becomes diverted around the Earth's magnetic obstacle, called the magnetosphere. Due to supersonic nature of the solar wind, a shock layer is formed at approximately 14 Earth's radii distance in front of the Earth. The understanding of the shock requires essentially multi-satellite observations, and so the Cluster mission is helping us understand the physics of shocks, but not only in front of the Earth's magnetosphere, but also elsewhere in the universe. One of the unique Cluster results is the direct measurement of electric currents in space. In the solar wind electric currents occur at shocks and such a measurement is shown in the figure below. At the bow shock the solar wind decelerates rapidly and at the same time the interplanetary magnetic field which is impeded in the solar wind stream becomes compressed. At the shock, strong electric currents flow, which can be measured with the four Cluster satellite for the first time; a typical current here is of the order of one million Amperes.

Top figure shows the components of three-dimensional current system flowing at the bow shock measured by four Cluster satellites, and the total magnetic field from Cluster 1, showing a strong compression of the interplanetary magnetic field. The bottom figure is a model of the current systems at the bow shock. (courtesy: Jonatan Eastwood, Imperial College, London)

The magnetopause is the interface between the subsonic solar wind (downstream of the bow shock) and the magnetosphere. The transfer of particles, momentum, and energy is still a widely debated issue, and the unique Cluster measurements are currently playing a key role in solving numerous related problems. One of the key processes is the magnetic reconnection that takes place both in the nightside magnetosphere leading to a colourful auroral display and in the dayside magnetopause causing transport of energy, momentum, and particles into the magnetosphere.

In the example below, taken place on 8 March 2003, the four Cluster spacecraft were moving outbound through northern near-noon magnetopause. Observations made by the 4 Cluster electron spectrometers and the 4 magnetometer during this crossing are shown in the figure below. Three of the spacecraft passed through the magnetopause at about 0653 UT (signified by the red dashed line) while the fourth, which lagged behind the other 3 by some 4500 km, remained within the magnetosphere. Those spacecraft that crossed the magnetopause observe a large change in the magnetic field traces in the lowest 4 plots (black trace for Cluster 1, red for cluster 2, green for Cluster 3, purple for Cluster 4). They also detect a change in the character of the electron populations, switching from a hot, low density nature (i.e. of magnetospheric origin) to a cold and dense one (i.e. of solar wind origin).

Electron energy flux (top four panels) from four Cluster satellites, and three magnetic field components and total magnetic field (bottom four panels). This figure shows traces of magnetic reconnection on the magnetopause (courtesy: Chris Owen, Mullard Space Science Laboratory, University College London)

Prior to crossing the magnetopause, all the spacecraft regularly detect a short-lived signature in the magnetic field data (some marked by the blue dashed lines in the figure). These are believed to be the signature of a magnetic "Flux Transfer Event" (FTE), resulting from a short burst of magnetic reconnection which results in a strip of the magnetosphere being ripped from its sunward facing edge by the Solar wind. After the crossing the interior spacecraft, Cluster 3 (green trace) continues to observe such events, while the 3 external spacecraft, Clusters 1, 2, and 4, observe different magnetic field and electron variations at similar times (marked by the orange lines). In particular, the electrons observed at the times of the transients have been heated and accelerated, confirming a magnetic reconnection origin for these events.

These two cases are examples of a long list of events encountered by the Cluster spacecraft while orbiting the near-Earth space. Many of them are related to such plasma processes as turbulence, particle acceleration, and reconnection, that are common not only in the Earth's magnetosphere, but also everywhere in the universe. However, the Earth's magnetosphere is the only place in the universe where we can study them in great detail with in-situ instruments. The achieved knowledge can later be applied to similar processes occurring in more remote and exotic places, such as the Sun, stars, galaxies etc.